A prion is an infectious protein, a concept derived from studies of scrapie of sheep, and of Kuru, and Creutzfeldt-Jakob disease of man. We discovered that the yeast Saccharomyces cerevisiae can be infected with two prions, two non-chromosomal genetic elements whose properties are those predicted for an infectious protein, not a nucleic acid replicon or a virus. The first, [URE3] is an altered form of Ure2p, the protein product of the chromosomal URE2 gene important in regulation of nitrogen metabolism. The second, [PSI], is an altered form of Sup35p, a subunit of the translation release factor and product of the chromosomal SUP35 gene. We showed that the N-terminal 65 aminoacid residues of Ure2p are sufficient, when overproduced, to induce the formation of [URE3] at >1000 fold the spontaneous frequency. Further characterization of the domains of Ure2p involved in prion generation shows that there are two non-overlapping parts of the protein, either one of which can alone induce this high frequency of [URE3] generation. As further evidence that [URE3] is a prion, we show that introduction of only a DNA plasmid carrying the URE2 gene into a yeast strain lacking the [URE3] element and unable to propagate it, makes the strain able to propagate [URE3] and induces its formation. This is evidence of a type not yet achieved in the mammalian prion system. We have devised simple screening methods for prion-inducing and prion-curing agents in which effects on two prions can be tested in the same strain. We hope that these methods will, like the Ames mutagen (carcinogen) tests be useful for simply determining candidates for drugs to treat scrapie and its human equivalents. We have begun purification of Ure2p, and find that this protein has different chromatographic properties on DEAE-cellulose when isolated from normal or [URE3] strains. This is consistent with our earlier finding that Ure2p is more protease-resistant in extracts of [URE3] strains than of normal cells.